The present invention relates to a receiver device of a communication system and, more particularly, to a receiver device of a communication system that comprises a few subchannels such as DMT modulation and filterbank modulation subchannels, a multicarrier communication system, or a communication system in which a band is divided into independent narrow bands by means of multicarrier modulation in which each subband is independently modulated.
The bit error rate (BER) of a multicarrier communication system of filterbank modulation, DMT modulation or FMT modulation or the like can be improved by using a reception signal that includes distortion caused by Interchannel Interference (ICI). Interchannel interference ICI is produced as a result of a system malfunction or inevitable environment (loss of orthogonality between subchannels caused by a frequency offset or the like, for example) in a communication system, such as an OFDM-CDMA communication system, for example. This interchannel interference ICI arises due to leakage of spectral energy and, occasionally, leakage known as crosstalk between subchannels.
The turbo receiver of the present invention is based on a maximum posterior probability estimation algorithm that uses ICI. In this turbo receiver, information that is derived from a first subchannel following nonlinear processing refines the estimated maximum posterior probability of a second subchannel and, similarly, information that is derived from the second subchannel refines the estimated maximum probability of the first subchannel.
The main benefit of the turbo receiver of the present invention is that the behavior of ICI is treated as a zero average Gaussian distribution probability variable (the Gaussian approximation that is used in K. Sathananthan and C. Tellambura, “Probability of error calculation of OFDM system with frequency offset”, IEEE Trans. Commun. Vol. 49, No. 11, Nov. 2001, pp1884-1888, for example) adopts a finite state discrete Markov process model. In the case of such an ICI model, a simple Gaussian approximation is thought to be more realistic due to the quality of ICI. The turbo receiver of the present invention is based on a maximum posterior probability estimation algorithm. In this turbo receiver, information that is derived from the first subchannel following nonlinear processing refines the estimated maximum posterior probability of the second subchannel and, similarly, information that is derived from the second subchannel refines the estimated maximum probability of the first subchannel.
In the event of a reception judgment for a target subchannel, the BER improves as the number of subchannels delivering the crosstalk to be considered increases. However, as the number of subchannels to be considered increases, the complexity of the algorithm increases and, as a result, a trade-off exists between the BER and the complexity. Therefore, the present invention proposes a general constitution for a turbo posterior algorithm for a multicarrier communication system, that is, a general constitution that makes it possible to eliminate complexity, in a case where there is mutual interference between N subchannels.
(a) Relationship Between Frequency Offset and ICI
In a multicarrier communication system in which a band is divided into a plurality of subbands that are independent narrow bands and the transmission data of each subband is sent and received after being frequency-multiplexed, that is, in a multicarrier communication system using filterbank modulation, DMT modulation, FMT modulation, or the like, the selection of a filter set has traditionally been executed under the constraints of complete removal of intersymbol interference (ISI) and interchannel interference (ICI) and so forth.
In an ideal transmission channel in which there is no Doppler shift, no offset frequency between the transmitter and receiver, and no signal distortion, the constraint is error-free recovery of the transmission symbol by the receiver. However, the frequency offset that is produced in each channel as a result of inaccurate tuning of the oscillator or a Doppler shift causes BER degradation caused by spectral leakage or ICI.
A unique method for alleviating such BER degradation renders the frequency offset as small as possible and, more specifically, estimates the frequency offset to be within 1% of the subcarrier frequency interval. However, this method necessitates a precise frequency offset estimation and, when a multicarrier signal mixed with noise is received, there is the problem that the precision of the frequency offset estimation is degraded when the noise level is large. In addition, this method does not operate properly in a high-speed fading channel, that is, in a high-speed fading channel in which the Doppler shift is not constant with respect to the transmission symbol and varies with time.
When there is one subchannel delivering crosstalk.
When there is one subchannel delivering crosstalk, if the frequency offset (frequency offset that is normalized by means of the channel interval) a is zero in OFDM, the transmission function (gain/frequency characteristic) of a first subchannel provides infinite attenuation at the center frequency f2 of a second subchannel (dotted line) as shown by the solid line in FIG. 1. Further, the transmission function of the second subchannel similarly provides infinite attenuation at the center frequency f1 of the first subchannel. That is, if the frequency offset a is zero, ICI is not produced between adjacent subchannels. In other words, if the frequency offset is zero, the subchannels are orthogonal and ICI does not exist at all.
However, if the frequency offset a is zero, each of the spectra of the adjacent subchannels indicates a non-zero mutual gain in the target subchannel as is clear for a1, a2 in FIG. 2. That is, as shown in FIG. 2, when the frequency offset is not zero, ICI (crosstalk) is produced between subchannels.
FIG. 3 is a general model for a multicarrier communication system in which ICI exists. 1 and 2 are transmitter devices of the subchannels ch1 and ch2; 3 and 4 are receiver devices of each of the subchannels; 5 and 6 are the transmission channels of the respective subchannels; 7 and 8 are multipliers that multiply the crosstalk coefficients a1, a2 by subchannel signals D1 and D2 respectively; 9 and 10 are synthesizers that synthesize the crosstalk (ICI) from another channel subchannel with their own subchannel signals; 11 and 12 are noise synthesizers. In FIG. 3, the data transmitted on the subchannels ch1 and ch2 are statistically independent (without correlation) and are written as the intersubchannel crosstalk coefficients (coupling coefficients) a1 and a2. As is clear from FIG. 3, the signal from the first subchannel is leaked to the second subchannel according to the coupling coefficient a1 and the signal from the second subchannel is leaked to the first subchannel according to the coupling coefficient a2.
When there are two subchannels delivering crosstalk
Consider a target subchannel, a first adjacent subchannel that is disposed below the target subchannel and a second adjacent subchannel that is disposed above the target subchannel. FIGS. 4 and 5 illustrate the frequency response of three subchannels in a case where the frequency offset is zero (FIG. 4) and a case where the frequency offset is not zero (FIG. 5).
The signals of the center frequencies f1, f2, and f3 that correspond with the first, second and third subchannels respectively are indicated by the vertical arrows in FIGS. 4 and 5. In FIGS. 4 and 5, the subchannel number 0(ch0) indicates the target channel, the subchannel number −1(ch−1) indicates the subchannel that is located below the target subchannel on the frequency scale and the subchannel number +1(ch+1) indicates the subchannel that is located above the target channel on the frequency scale. Supposing that the cycle of a DMT symbol is T, the frequency scale is normalized with a channel interval equal to 1/T. That is, one unit of the frequency scale is a channel interval. As shown in FIG. 4, when the frequency offset (normalized with the channel interval) a is 0, the transmission function of the lower subchannel and the upper subchannel indicated by the solid line A and dashed line B in FIG. 4 respectively provides infinite attenuation at the center frequency f2 of the target subchannel (dotted line C). Further, the transmission function of the target subchannel similarly provides infinite attenuation at the center frequencies f1 and f3 of the lower and upper subchannels. That is, if the frequency offset a is zero, ICI is not produced between adjacent channels. In other words, if the frequency offset is zero, the subchannels are orthogonal and ICI does not exist at all.
However, when the frequency offset a is not zero, the orthogonality of the subchannels breaks down and ICI is produced. FIG. 5 shows the spectral characteristic of each subchannel when the frequency offset a is not zero in the DMT system. Crosstalk from the subchannels Ch−1 and Ch+1 to the target subchannel Ch0 has a non-zero mutual gain that is denoted as a−10 and a10 in FIG. 5. In this notation, the first index of a denotes the subchannel constituting the interference source and the second index denotes the subchannel that is the interference target. As mentioned earlier, when the frequency offset a is not zero, non-zero mutual gain, that is, ICI (crosstalk) is produced between subchannels.
FIG. 6 is a general model serving to illustrate the mutual ICI of three subchannels in a DMT system with a frequency offset. 111, 112, and 113 are the transmitter devices of the subchannels ch−1, ch0 and ch+1; 121, 122, and 123 are receiver devices of the respective subchannels; 131, 132, and 133 are transmission channels of the respective subchannels; 14ij is a multiplier that multiplies a transmission coefficient (interference coefficient) aij for leakage from a number-i subchannel to a number-j subchannel by a subchannel signal Di; 151, 152, and 153 are synthesizers that synthesize crosstalk (ICI) from another subchannel with their own subchannel signals; and 161, 162, and 163 are noise synthesizers.
As is clear from FIG. 6, the signal from the lower subchannel ch−1 leaks to the target subchannel ch0 via the crosstalk coefficient a−10 and the signal from the upper subchannel ch+1 leaks to the target subchannel via the crosstalk coefficient a10. Further, the noise components that are written as n1(t), n2(t), and n3(t) in FIG. 3 are statistically independent (without correlation) due to the frequency orthogonality between the subchannels.
When there are four subchannels delivering crosstalk
Consider a target subchannel, two adjacent subchannels that are disposed below the target subchannel and two adjacent subchannels that are disposed above the target subchannel. FIGS. 7 and 8 illustrate the frequency response of five subchannels Ch−2 to Ch+2 in a case where the frequency offset is zero (FIG. 7) and in a case where the frequency offset is not zero (FIG. 8).
The signals at the center frequencies f1 to f5 that correspond with the first to fifth subchannels respectively are indicated by the vertical arrows in FIGS. 7 and 8. In FIGS. 7 and 8, the subchannel number Ch0 denotes the target channel. When a is 0, the transmission function of the other subchannel provides infinite attenuation at the center frequency f3 of the target subchannel Ch0. In other words, if the frequency offset is zero, the subchannels are orthogonal and ICI does not exist at all.
However, when the frequency offset a is zero, the orthogonality of the subchannels breaks down and ICI is produced. FIG. 8 shows the spectral characteristic of each subchannel when the frequency offset a is not zero in the DMT system. Crosstalk from the subchannels Ch−2, Ch−1, Ch+1, and Ch+2 to the target subchannel Ch0 has a non-zero mutual gain that is denoted as a−20 a−10, a10 and a20 in FIG. 8.
FIG. 9 is a general model serving to illustrate the mutual ICI of five subchannels in a DMT system with a frequency offset. 211 to 215 are transmitter devices of the subchannels ch−2 to ch+2; 221 to 225 are receiver devices of the respective subchannels; 231 to 235 are transmission channels of the respective subchannels; 24ij is a multiplier that multiplies a transmission coefficient (interference coefficient) aij for leakage from a number-i subchannel to a number-j subchannel by a subchannel signal Di; 251 to 255 are synthesizers that synthesize crosstalk (ICI) from another subchannel with their own subchannel signals; and 261 to 265 are noise synthesizers.
As is clear from FIG. 9, the signals from the lower subchannels ch−2 and ch−1 leak to the target subchannel ch0 via the crosstalk coefficients a−20 and a−10 and the signals from the upper subchannels ch+2 and ch+1 leak to the target subchannel via the crosstalk coefficients a10 and a20. Further, the noise components that are written as n1(t) to n5(t) in FIG. 9 are statistically independent (without correlation) due to the frequency orthogonality between the subchannels.
(b) Technical Problems
As mentioned earlier, even when ICI is produced, it is necessary to be able to determine correctly the values (code in the case of binary numbers) of the reception signals and transmission information symbols of each subchannel. For this reason, the present inventors propose a receiver device (turbo receiver) that implements a turbo posterior algorithm to enhance BER by using ICI when there is one subchannel delivering crosstalk to the target subchannel (ICI-2 case) and when there are two subchannels (ICI-3 case). Further, although the BER can be improved as the number of subchannels to be considered is increased, the algorithm grows complicated and the constitution of the turbo receiver implementing the algorithm becomes complex. That is, a trade-off exists between BER and the number of subchannels to be considered and there are limitations in enhancing BER by adding the number of subchannels to be considered.